1. Field of the Invention
The present invention relates to an information recording medium and a recording method using the same.
2. Description of the Related Art
A conventional recordable/erasable information recording medium includes a substrate, a first protective layer, a recording layer, a second protective layer, and a reflecting layer.
When a recording/erasing operation is to be performed using such an information recording medium, a light beam is radiated on the entire surface of the information recording medium to heat the information recording medium at a temperature lower than the melting point of the material of the recording layer, thereby setting the material of a recording layer in a high crystallinity (a state in which atoms are relatively regularly arranged; to be referred to as a crystalline state hereinafter). Strong pulse light having a short wavelength is radiated on the information recording medium to heat and melt the recording layer, and the recording layer is rapidly cooled. In this manner, part on which the pulse light is radiated has a low crystallinity (a state in which an atomic arrangement is disturbed; to be referred to as an amorphous state hereinafter).
As described above, since the crystalline and amorphous states have different atomic arrangement structures, these states have different optical characteristics such as transmittances or reflectances. Information can be recorded by using the difference between the optical characteristics. Information recorded as described above can be erased as follows. That is, weak pulse light having a long wavelength is radiated on the recorded portion to heat the recorded portion to a temperature which is equal to or lower than the melting point of the material constituting the recording layer, and the recorded portion is gradually cooled. This is because the state of the material of the recorded portion is returned to the original state, i.e., the crystalline state.
In an actual information recording medium, a change in reflectance between the crystalline state and the amorphous state is used as a signal as described above. For this reason, the thickness of each layer is designed in consideration of the optical interference effects of the interface between the protective layer and the recording layer and the interface between the protective layer and the reflecting layer. Therefore, according to the optical constants of the materials used in the information recording medium, there are optimal thicknesses capable of obtaining a large change in reflectance.
The following fact is known. That is, when the thickness of the protective layer increases, heat flowing from the recording layer to the reflecting layer is interfered, and rapid cooling cannot be satisfactorily controlled by modulation of a laser power, thereby degrading recording characteristics. Therefore, in a conventional technique, when GeSbTe or the like is used as the material of the recording layer, the thickness of the protective layer is set to fall within a range of 100 to 200A, thereby performing recording such that the reflectance decreases (T. Ohta et al. JJAP. Vol. 128 (1989) SUPPLEMENT 28-3, pp. 123-128).
As a recording method using such an information recording medium described above, mark position recording and mark length recording are known. That is, recording marks having the same shape are formed, and information is obtained by intervals between the centers of the recording marks. In the mark length recording, recording marks having lengths corresponding to information are formed, and information is obtained by the lengths of the recording marks.
In a conventional information recording medium for mark position recording, the size of a recorded portion (amorphous area) is not larger than that of a non-recorded portion (crystalline area) on a recording layer. On the other hand, in an information recording medium for mark length recording, the size of a recorded portion is larger than that of the recorded portion obtained in the mark position recording. For example, assume that the diameter of the recording mark is 0.78 .mu.m, and that recording is performed at the same density in mark position recording and mark length recording. In this case, a ratio of the area of the recorded portion to the area of the non-recorded portion on the recording layer in the mark position recording is 26%, and a ratio of the area of the recorded portion to the area of the non-recorded portion on the recording layer in the mark length recording is 44%. When these recording methods are applied to a conventional information recording medium in which recording is performed such that the reflectance decreases, as the ratio of the area of the recorded portion to the area of the non-recorded portion is larger, an average reflectance obtained during a reproducing operation is lower than that obtained before a recording operation is performed.
In the mark position recording, even when recording is performed such that the reflectance decreases, a certain amount of reflected light can be obtained, and focusing and tracking operations are performed by a normal optical disk drive to reproduce a signal. However, in an information recording medium in which recording is performed such that the reflectance decreases, when recording marks are formed at a high density in mark length recording in accordance with high-density recording, a ratio of the area of the recorded portion to the entire area of the recording layer increases in a reproducing operation. For this reason, the average reflectance decreases to 60% or less of the original average reflectance. In general, in order to stably perform focusing and tracking operations, at least a reflectance of about 10% is required. Therefore, in this case, an amount of reflected light required for the focusing and tracking operations cannot be obtained. In order to increase the reflectance, the thickness of the second protective layer must be increased. In this case, a large reflectance change amount cannot be obtained.
When an information recording medium in which recording is performed such that the reflectance decreases is used, if light does not escape from the lower surface of the reflecting layer, the absorbance of the recorded portion increases, and the absorbance of the non-recorded portion decreases accordingly. For this reason, an absorbance obtained when a new mark is overwritten in the recorded portion is different from an absorbance obtained when a new mark is overwritten in the non-recorded portion, and the rates of increase in temperature of the recorded and non-recorded portions of the recording layer are different from each other during a recording operation. In addition, latent heat is required to melt the non-recorded portion because the state of the non-recorded portion is a crystalline state. For this reason, when the recorded and non-recorded portions are heated by the same laser power, the difference between the rates of temperature increase in temperature further increases. For this reason, the sizes of formed recording marks vary depending on areas in which the recording masks are formed. Therefore, a recording scheme in which the edge portions of recording marks have information is used, the edge portions fluctuate depending on their positions on the recording layer.